ML24215A241
| ML24215A241 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 08/02/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24215A000 | List:
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| References | |
| LO-169995 | |
| Download: ML24215A241 (1) | |
Text
Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-13 Receipt Date: 12/04/2023 Question:
During review of the loss of feedwater event (described in EC-116269 R0), (( 2(a),(c),ECI flow is terminated much earlier and long before FWIV closure. Terminating the feedwater flow completely before the FWIV closes in the case results in DHRS inventory that is more optimum for RCS cooling than if feedwater flow were to continue ((
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2(a),(c),ECI This is more important for transients where only 1 train of DHRS is available. The DHRS performance is closely tied to loop inventory, and it is not clear if NuScale has accounted for this deterioration in DHRS performance. Please evaluate and address all applicable LOCA and non-LOCA analyses to confirm that adequate modeling has been applied for DHRS performance in this regard. NuScale Nonproprietary NuScale Nonproprietary
Response
EC-116269, NPM-20 Loss of Normal Feedwater Transient Analysis, Revision 0, was provided to the NRC in the electronic reading room (eRR) as part of the response to audit question A-15.2.7-2. EC-116269 provides the NRELAP5 analysis supporting final safety analysis report (FSAR) Section 15.2.7 for the US460 standard design approval application (SDAA). The initiating event being modeled is a loss of feedwater flow. ((
}}2(a),(c),ECI The NRC staff observation that feedwater flow is terminated much earlier than feedwater isolation valves (FWIVs) close is correct. However, the reason feedwater flow is terminated early is that termination of feedwater flow is the modeled initiating event. (( }}2(a),(c),ECI Therefore, feedwater flow is zero before the secondary system valves begin to change positions.
The NRC observation that potential continuing feedwater flow until FWIVs close will change the secondary side inventory and therefore impact DHRS performance is correct. However, the NuScale Nonproprietary NuScale Nonproprietary
purpose ((
}}2(a),(c),ECI is to maximize RCS pressure, not to study the effects of DHRS. None of the cases associated with EC-116269 are informative for studying the effects of high secondary inventory on DHRS behavior because of the loss of feedwater flow as the initiating event; other Chapter 15 initiating events are more appropriate for investigating this concern.
The increase in feedwater flow event in FSAR Section 15.1.2 is an event where secondary side inventory increases. Due to the potential impact of high inventory on DHRS performance, the increase in feedwater flow analysis explicitly addresses high inventory cases. As described in FSAR Section 15.1.2.3.3: Steam generator overfill cases are evaluated with initial conditions including primary flow and RCS temperature biased to maximize SG level. Boundary conditions such as pool temperature are biased to minimize DHRS heat removal, high decay heat is assumed, the maximum FW pump curve is modeled, the maximum containment isolation valve closure time is assumed, and single failure of one FWIV to close is assumed. The SG level calculated during an increase in FW flow demonstrates the SG does not overfill as shown in Figure 15.1-18. Evaluation of cases biased for SG overfill demonstrate adequate DHRS heat removal capability is available under these conditions as shown in Figure 15.1-17. The analysis supporting FSAR Section 15.1.2 is found in EC-0000-8328, NPM-20 Increase in Feedwater Flow Analysis, Revision 0. EC-0000-8328 was provided to the NRC in the eRR as part of the response to audit question A-15.1.1-1. ((
}}2(a),(c),ECI NuScale Nonproprietary NuScale Nonproprietary
In addition to considering the impact of inventory on DHRS performance for events like the increase in feedwater flow event, FSAR Chapter 15 also includes events that disable an entire train of DHRS. The DHRS is designed to be able to perform its function even in the event of a single failure. As a result, the single failure assumptions in most Chapter 15 analyses can impact DHRS performance (e.g., loop inventory is altered by assuming single failure of an FWIV that delays loop isolation until the feedwater regulating valve closes), but do not disable an entire train. However, in the event of a steam system piping failure in FSAR Section 15.1.5 or a feedwater system pipe break in FSAR Section 15.2.8, the initiating event is a break that, depending on the location, can disable an entire DHRS train. For example, FSAR Section 15.1.5.2 states: The DHRS is a safety-related system credited to actuate and mitigate the effects of this transient. The redundancy and passive nature of the DHRS ensure the system performs its intended function despite a single failure. The operation of a DHRS train is challenged when an SLB is located on the steam line downstream of the MSIV, but upstream of the secondary MSIV, with a single failure of the MSIV, or when an SLB occurs inside containment. These scenarios prevent one train of the DHRS from functioning by preventing isolation of one SG train. Similar to the increase in feedwater flow event, the steam system piping failure calculation methodology uses a maximum feedwater flow rate ((
}}2(a),(c),ECI NuScale agrees that DHRS performance is related to loop inventory and has therefore accounted for this effect in the analysis of relevant initiating events. Similarly, NuScale agrees that operation of DHRS with only a single train is more limiting for decay heat removal than when two trains are available. Although a single failure alone does not disable an entire train of DHRS, the safety analysis in Chapter 15 properly considers initiating events that can directly, or in combination with a single failure, result in failure of an entire train of DHRS. NuScale confirms that adequate modeling has been applied in the LOCA and non-LOCA transients for DHRS performance in this regard.
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Additionally, consistent with the non-LOCA methodology previously approved by the NRC during the design certification application (DCA), TR-0516-49416-P-A Revision 3, no penalties, biases, or uncertainties are applied to the DHRS heat transfer. The sensitivity studies performed during the DCA review that considered biases on the heat transfer demonstrated no impact on the figures of merit. The associated safety evaluation report (SER) included a condition and limitation to confirm the sensitivity study results if the design changed in the future. In lieu of additional sensitivity studies, the current non-LOCA methodology topical report includes additional DHRS testing scaled to the SDAA design (i.e., addresses the design changes from DCA). The testing results demonstrate that NRELAP5 can predict DHRS heat transfer for the SDAA design with reasonable to excellent agreement, and therefore without the need to apply a bias or uncertainty in the NPM plant calculations. ((
}}2(a),(c),ECI No changes to the SDAA are necessary.
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